Causing people to stumble is a metaphor for offending them. Possible meanings are 1) Jesus was concerned that the tax collectors would think he had done something very wrong. Alternate translation: "so that we do not cause the tax collectors to think that we do something that is very bad" or 2) Jesus was concerned that the tax collectors might think that he was a sinner and so could not be the Messiah. Alternate translation: "so that we do not cause the tax collectors to reject me" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here "you" is singular and refers to Peter. Each man had to pay a half shekel tax. So one shekel would be enough for Jesus and Peter to pay their taxes. (See: [[rc://en/ta/man/jit/figs-you]])